Perhaps the most common hearing damage today takes the form of defects in the inner ear which usually are corrected with different types of hearing aids attached to the external ear canal. Certain patients who need hearing aids cannot use conventional hearing aids which transmit air-borne sound through that canal but instead must use a bone-conduction hearing apparatus. The reasons can be several. For example the hearing canal, can be defective, perhaps already from birth, so that the air-borne sound is not passed while the organs of the inner ear are intact. Experience has shown that the inner ear is very sensitive to sound in the form of vibrations which are transmitted through the skull. A transmitter which generates sound vibrations is then attached to the side of the head, for example using a steel spring over the head, in the region of the ear and transmits sound via the head skin and bone to the inner ear. The pressure with which the transmitter is applied against the head in order to obtain as good mechanical contact as possible is uncomfortable for the patient. In addition the skin and tissue cause damping of the vibrations especially in the high frequencies. During recent years through improved medical techniques one can permanently implant a metallic skin penetration which is anchored in the skull for transmission of sound by vibrations. Because no damping skin is present between the sender and the skull a considerably better sound transmission is obtained while simultaneously the electrical current used by the sender can be reduced. In addition esthetic improvements have been obtained because the steel spring over the head is not needed.
A known coupling is available on the market today and is shown in FIG. 1. It includes ball-shaped and cup-shaped coupling parts of which one, for example the ball-shaped part, is attached to the skull behind the ear of the patient. The coupling part penetrates the skin and sticks out 4-6 mm outside the skin. To that coupling part the sender with its cup-shaped coupling part is attached with a "snap lock."
This known coupling has a number of drawbacks. For example the ball-shaped coupling part sticking out of the head causes several inconveniences. In addition to purely esthetic drawbacks clothing can easily catch on the ball. In spite of the fact that the part of the coupling in contact with tissue (skull bone, skin) is made of titanium, which has shown very good characteristics concerning ingrowth with the bone, the skull bone can be damaged by strong mechanical loading such as blows and the like. The problem is accentuated by the fact that the coupling comprises a not inconsiderable lever by virtue of its large physical height. This is also a problem during sleep when the coupling can be in the way or catch on the bedclothes.
Other problems, such as that the coupling unscrews itself and falls out, have appeared during practical use. For example experience has shown that the mechanical contact between the coupling parts is weakened in time which results in reduced sound quality in the transmission. This is because distortion results at certain frequencies and sound strengths and the coupling parts are not able to maintain the mechanical contact. Further drawbacks are that the coupling is not provided with a "neutral position" which means that the transmitter, which is often asymmetrically formed, can slip around the contact ball and lie against the skin which can cause interference and discomfort for the patient.